1. Field of the Invention
The present invention relates to a negative ocular (an inner-focusing type eyepiece lens system) with two lens elements, which is used in combination with an objective lens system of a telescope or binoculars.
2. Description of the Related Art
In a two-lens-element eyepiece lens system having a lens element on the user's eye side (hereinafter, an eye-side lens element) and a field lens element on the object side, some types of eyepiece lens systems are known in the art, such as a Ramsden eyepiece lens system (hereinafter, the R-type), a Huygenian eyepiece lens system (hereinafter, the H-type), and a Mittenzwey-Huygenian eyepiece lens system (hereinafter, the MH-type) in which the field lens element of the H-type is formed into the shape of a meniscus lens element. The R-type eyepiece lens system is an outer-focusing eyepiece lens system in which the eye-side lens element and the field lens element are provided with the same amount of positive power, and are symmetrically positioned. An intermediate image formed by an objective lens system is positioned (formed) at the outer side (the object side) of the field lens element. Moreover, in the R-type eyepiece lens system, lateral chromatic aberration can be corrected by the following condition:
f.sub.1 :f.sub.2 :L=1:1:1 PA1 wherein PA1 f.sub.1 designates the focal length of the eye-side lens element; PA1 f.sub.1 :L:f.sub.2 =1:2:3; or PA1 f.sub.1 :L:f.sub.2 =2:3:4 PA1 wherein PA1 f.sub.1 designates the focal length of the eye-side lens element; PA1 f.sub.2 designates the focal length of the field lens element; and PA1 wherein EQU S=.nu..sub.1 /.nu..sub.2 ; EQU P=f.sub.2 /f.sub.1 ; PA1 f.sub.1 designates the focal length of the eye-side lens element; PA1 f.sub.2 designates the focal length of the field lens element; PA1 .nu..sub.1 designates the Abbe number of the eye-side lens element; PA1 .nu..sub.2 designates the Abbe number of the field lens element; and PA1 wherein PA1 r.sub.3 designates the radius of curvature of the eye-side surface of the field lens element; and PA1 r.sub.4 designates the radius of curvature of the object-side surface of the field lens element.
f.sub.2 designates the focal length of the field lens element; and
L designates the distance between the principal points of the eye-side lens element and the field lens element.
It is noted that the inner-focusing type eyepiece lens system is hereinafter referred to as a negative ocular in which the intermediate image is formed between the field lens element and the eye side lens element; and the outer-focusing eyepiece lens system is hereinafter referred to as a positive ocular in which the intermediate image is formed outside of the field lens element and the eye side lens element.
The H-type eyepiece lens system is a negative ocular in which the eye-side lens element and the field lens element are planoconvex lens elements which are made from the same lens material, and the convex surfaces face towards the object. An intermediate image formed by an objective lens system is positioned (formed) between the eye-side lens element and the field lens element. Moreover, in the H-type eyepiece lens system, lateral chromatic aberration can be corrected by the following condition:
L designates the distance between the principal points of the eye-side lens element and the field lens element. ("Telescopic Optics & Refraction for Amateur Astronomers" by Yoshida Shotaro)
In the MH-type eyepiece lens system, the field lens element of the H-type is formed into the shape of a meniscus lens element so that a field of view becomes wider than that of the H-type. On the other hand, the conditions for correcting lateral chromatic aberration are the same as those of the H-type.
In the R-type eyepiece lens system, since the field lens element is positioned where the intermediate image formed by the objective optical system is formed, dust and other foreign matter tend to be noticeable. Consequently, the field lens element and the eye-side lens element can not be positioned to satisfy the conditions for correcting lateral chromatic aberration. In the H-type eyepiece lens system, and the HM-type eyepiece lens system, the eye relief (the distance from the eye-side surface of the eye-side lens element to the eyepoint) is about 0.2 f to 0.3 f (f: the focal length of the entire eyepiece lens system), which is extremely short. Therefore in an eyepiece lens system with a short focal length, it is very difficult for a user to obtain a clear image.